1. Field of the Invention
The present invention is generally related to methods of forming a mask for controlling material deposition, and more particularly to methods of forming a print-patterned mask for plating and the like having high aspect ratio mask structures with controlled side-wall profiles.
2. Description of the Prior Art
Digital inkjet lithography is a maturing technology designed to reduce the costs associated with photolithographic processes, used often in the fabrication of micro-electronic devices, integrated circuits, and related structures. Digital lithography directly deposits material in desired patterns onto a substrate, taking the place of the delicate and time-consuming photolithography processes used in conventional device manufacturing. One application of digital lithography is the formation of a mask (referred to herein as a “print-patterned mask”) for subsequent processing (e.g., plating, etching, implanting, etc.)
Typically, digital lithography involves depositing a print material by moving a print head and a substrate relative to one another along a primary axis (the “print travel axis”). Print heads, and in particular, the arrangements of the ejectors incorporated in those print heads, are optimized for printing along this print travel axis. Printing takes place in a raster fashion, with the print head making “printing passes” across the substrate as the ejector(s) in the print head dispense individual “droplets” of print material onto the substrate or other previously deposited material. Typically, the print head moves relative to the substrate in each printing pass, but the equivalent result may be obtained if the substrate is caused to move relative to the print head (for example, with the substrate secured to a moving stage) in a printing pass. At the end of each printing pass, the print head (or substrate) makes a perpendicular shift relative to the print travel axis before beginning a new printing pass. Printing passes continue in this manner until the desired pattern has been fully printed onto the substrate.
Materials typically printed by digital lithographic systems include phase change material and solutions of polymers, colloidal suspensions, such suspensions of materials with desired electronic properties in a solvent or carrier. For example, U.S. Pat. Nos. 6,742,884 and 6,872,320 (each incorporated herein by reference) teach a system and process, respectively, for printing a phase change material onto a substrate for masking. According to these references, a suitable material, such as a stearyl erucamide wax, is maintained in liquid phase over an ink-jet style piezoelectric print head, and selectively ejected on a droplet-by-droplet basis such that droplets of the wax are deposited in desired locations in a desired pattern on a layer formed over a substrate. The droplets exit the print head in liquid form, then solidify after impacting the layer, hence the material is referred to as a phase-change material.
Once dispensed from an ejector, a print material droplet attaches itself to the surface through a wetting action as it solidifies in place. In the case of printing phase-change materials, solidification occurs when a heated and liquefied printed droplet loses its thermal energy to the substrate and/or environment and reverts to a solid form. In the case of suspensions, after wetting to the substrate, the carrier most often either evaporates leaving the suspended material on the substrate surface or the carrier hardens or cures. The thermal conditions and physical properties of the print material and substrate, along with the ambient conditions and nature of the print material, determine the specific rate at which the deposited print material transforms from a liquid to a solid, and hence the height and profile of the solidified deposited material.
If two adjacent droplets are applied to the substrate within a time prior to the solidification of either or both droplets, the droplets may wet and coalesce together to form a single, continuous printed feature. Surface tension of the droplet material, temperature of the droplet at ejection, ambient temperature, and substrate temperature are key attributes for controlling the extent of droplet coalescence and lateral spreading of the coalesced material on the substrate surface. These attributes may be selected such that a desired feature size may be obtained.
It is known to print droplets one atop another in order to build up the height of a structure above the substrate. This is particularly relevant when printing a mask, such as for plating, deposition of a color pixel filter, etc., where a high aspect ratio structure is desired or required. However, we have discovered that when printing one droplet atop another there is a tendency for the first droplet printed directly onto the substrate to spread out laterally upon printing. When a second droplet is printed onto the first, the extent of the lateral spreading of the second droplet is less than that of the first. This is illustrated in FIG. 12, in which a substrate 10 has a plurality of printed features 12a, b deposited directly thereon, and a plurality of features 14a, b printed atop of features 12a, b. Each feature 12a, b is initially deposited onto substrate 10 by a print head 18. Following the ejection of a droplet forming feature 12a in a first position (a), print head 18 advances in the print travel direction (PT) to a second location (b) and deposits a next droplet to form feature 12b. This process may be repeated such that with print head 18 in position (a), feature 14a is formed over feature 12a, print head 18 advanced in the print travel direction to location (b), and feature 14b then deposited over feature 12b, and so on.
Print head 18 ejects droplets 20 in a direction indicated by arrow P towards substrate 10. Typically, direction P is roughly perpendicular to the plane of the top surface 22 of substrate 10. Droplets generally form symmetrical features when deposited, each feature having a centerline Ca, Cb, etc., representing effectively a line of symmetry through the feature. Due to the stepped nature of the print head movement, centerlines of features formed from droplets deposited at position (a) tend to be collinearly aligned, centerlines of features formed from droplets deposited at position (b) tend to be collinearly aligned, and so forth.
As can be seen from FIG. 12, the lateral width (parallel to the plane of surface 22) of features 12a, 12b exceed the lateral width of features 14a, 14b. This is true even though the volume of droplet material, the temperature of the droplets at ejection from print head 18, the rate of droplet ejection, and other details for features 14a, 14b were the same as for the deposition of features 12a, 12b. However, the centerlines of the stacked features tend to remain collinear, as shown in FIG. 12.
The observed difference in lateral width of the deposited droplets leads to the consequence that the sides of the built-up layers of features taper away from a line perpendicular to the plane of surface 22. This is illustrated by the lines S1 and S2, representing the angle of the sidewalls of the built up feature stacks 12a/14a, and 12b/14b, respectively. It is noted that since the centerlines of the feature stacks are roughly perpendicular to the plane of surface 22, the plane of the sidewalls of those feature stacks are each are inclined (e.g., by angles α, β, respectively) with reference to the respective centerlines Ca, Cb.
It is desirable when forming a mask structure, for example a plating mask, that the sidewalls of the mask features be nearly perpendicular to the surface of the substrate on which they are formed. For example, the bus connections and metal contacts over silicon or other photosensitive material for a solar cell extend over virtually the entire collection surface area for maximum capture and conduction of electrons produced by photovoltaic effects. However, the total area covered by the collection electrodes should be minimized so that they block as little light as possible from reaching the p/n junction layer of the solar cell. That is, increasing the ratio of unmasked surface to masked surface increases the conversion and efficiency of the cell. For this reason, there is a desire to form these electrodes as narrow as possible, on the order of 50 to 100 microns (micrometers), and provide sufficient conductivity by forming them to be relatively tall (that is, they are formed to have a high aspect ratio). Thus, bearing the above description of prior art digital lithographic mask formation in mind, when forming a mask for plating a structure there is a need to optimize the sidewall angle of feature stacks with reference to a plane which is perpendicular to the substrate surface.